The present invention relates to the extrusion of chocolate and other fat-containing confectionery materials and more particularly to a process for the continuous extrusion of solid or semi-solid chocolate and other fat-containing confectionery materials using a screw extruder.
In European Patent Application Publication No. 0 603 467, which sets forth the contents of Application No. 93114251.7, the entirety of which is hereby incorporated by reference, describes a process for plastically extruding a fat-containing confectionery material which comprises feeding the fat containing confectionery material into an extruder and applying pressure to the fat-containing confectionery material in a substantially solid or semi-solid nonpourable form upstream of a flow constriction whereby the temperature, pressure, contraction ratio and extrusion rate are such that the fat-containing confectionery material is extruded and remains in a substantially solid or semi-solid nonpourable form to produce an axially homogeneous extruded product having a cross section that is of substantially the same profile as the die exit of the extruder, which is capable of retaining its shape and which has a temporary flexibility, or plasticity, enabling it to be physically manipulated, cut or plastically deformed before losing its flexibility or plasticity.
The temporary flexibility of the extruded fat-containing confectionery material obtained in the process of the above-mentioned co-pending patent application may last for up to 4 hours or more, e.g. from 1 second or less to 2 hours, e.g. from 10 seconds to 1 hour. During this period of temporary flexibility, the extruded fat-containing confectionery material may be cut cleanly, and this is in contrast to fat-containing confectionery material extruded by a process as described in the afore-noted European '467 Application after the temporary flexibility has been lost, or to a normal set chocolate which has to be heated slightly for cutting which otherwise would shatter owing to its brittleness.
As described in the afore-noted European '467 Application, a chocolate or other fat-containing confectionery material may be fed into the barrel of an extruder in a liquid or paste form, but it preferably is fed into the barrel in a solid or semi-solid non-pourable form. The chocolate or fat-containing confectionery material may be in a granular or continuous form. When in granular form, the granular nature of the material appears to be lost during extrusion to give an essentially uniform material.
Additionally, it is described in the European '467 Application that during extrusion, it is important that the fat-containing confectionery material does not become pourable and that the extrusion temperature and pressure should be maintained below a level where this may happen. Additionally it is described that the extrusion pressure is partially dependent, amongst other things, on the contraction ratio, the extrusion temperature and the confectionery composition and may be from 1 to 1000 bars, e.g., from 5 to 500 bars and typically from 5 to 250 bars, and it is described that an important feature of the extrusion process is that for a given die configuration and material composition, the extrusion rate is weakly dependent upon the extrusion pressure.
The extrusion process may be carried out batchwise or continuously, and some examples of advantages of continuous extrusion are:
a) the extrusion rate is constant and uninterrupted, and
b) coextrusion is simpler because the chocolate or fat-containing confectionery material is extruded at a constant rate, and
c) continuous downstream cutting of the extruded flexible chocolate can be carried out in order to form sectioned pieces.
However, when carrying out continuous extrusion using a screw extruder, heat may be generated by the friction of the shear of the chocolate between the screw and the barrel wall. This heat tends to raise the temperature so that the chocolate melts, and the liquefied fat of the melted chocolate acts as a lubricant causing the chocolate to slip against the barrel wall which prevents it from extruding efficiently. It is important that the chocolate should stick sufficiently at the surface of the barrel wall and slip sufficiently at the surface of the extruder screw in order that the extruder can generate an extrusion pressure. It is equally important that the temperature at either surface is not sufficiently high to cause substantial melting of the chocolate.